The present invention relates generally to radio frequency (RF) signal combiners, and more particularly to a multiport RF signal combiner for combining a plurality of RF signals for transmission by a single antenna.
In radio systems, it is often desirable to combine a number of RF signals so that they may be transmitted by a single antenna. Separate antennas may be used for each RF signal provided each antenna is spatially isolated with respect to the other. However, when it is necessary to transmit three or more RF signals from one site, the use of separate, spatially isolated antennas, becomes impractical.
In order to combine a number of RF signals from radio transmitters and couple them to a common antenna, each transmitter must be isolated from one another to prevent intermodulation and possible damage to the transmitters. Two types of conventional combiners have been utilized in radio systems for combining RF signals from a number of transmitters. One type of combiner utilizes tunable devices, such as the hybrids and duplexers described in an article by William B. Bryson, entitled "Antenna Systems and Transmitter Combiners", Part III, published in Communications, Jan. 1981, pages 44-46, 48-50, 79, 80, and 82. These tunable devices typically accept two RF signals which are combined to provide a common output. In order to combine more than two RF signals, the tunable devices must be cascaded. Thus, not only do these tunable devices require precise manual tuning, but also incur additional RF signal losses and expense when cascaded to accommodate three or more radio signals.
Another type of conventional radio signal combiner typically includes an isolator and cavity filter for each RF signal transmitter and a combiner for interconnecting the RF signals from each of the cavity filters. However, the combiner in such combining systems terminates each transmitter with a capacitive discontinuity. The capacitive discontinuity can be alleviated to some degree by connecting the output of the combiner to an impedance adjuster, utilizing precisely located stubs for cancelling the capacitive discontinuity. One such impedance adjuster is described in an article entitled "Transmitter Multiplexing System in UHF Mobile Radio", by K. Uenishi, K. Araki and H. Ishii, published in the IEEE Transactions in Vehicular Technology, Vol. VT-18, No. 1, May, 1969, at pp. 1-11. However, the use of an impedance adjuster does not adequately cancel the capacitive discontinuity introduced by the combiner, since the impedance adjuster is located on the output transmission line, physically displaced from the actual interconnection. Thus, neither of the foregoing conventional combiners is suitable for combining three or more RF signals, while minimizing RF signal attenuation and capacitive discontinuities.